Control system for pwm inverter

A control system for controlling a PWM inverter (4) so that the inverter drives an induction motor (6) at a variable voltage and a variable frequency while keeping constant a ratio of the output voltage of the inverter to the output frequency thereof, is disclosed which includes current component detecting means (51, 52, 7, 8, 9) for detecting an effective current (it) corresponding to a load, from the input current of the induction motor, frequency setting means (113, 11) for setting the output frequency of the PWM inverter, flux setting means (112) for setting the amount of magnetic flux generated in the induction motor, correcting frequency calculating means (13, 14, 164) for calculating a frequency correcting quantity (ωs) proportional to the effective current, and correcting-flux calculating means (102) for calculating a flux correcting quantity proportional to the effective current. In a case where the detected effective current is judged to be less than a predetermined value, the frequency correcting quantity is added to the set output frequency to obtain an output frequency command, and the sum of the flux correcting quantity and the set amount of magnetic flux is multiplied by the value of the output frequency command to obtain an output voltage command, for the purpose of controlling the operation of the PWM inverter on the basis of the output frequency command and the output voltage command.